Ambient light sources supplied by means of a power line often deliver a luminance having periodic fluctuations, the frequency of which is correlated with a line frequency of typically 50 Hz or 60 Hz. Conventional methods for determining the frequency are based on measurements with fast sampling and an evaluation of the measurements by means of fast Fourier transform. However, such methods are computationally very complex and often have low resolution due to the fast sampling. Furthermore, such methods require a high signal strength of the ambient light due to the short integration time.